1. Field of the Invention
The present invention relates to a cantilever displacement detection apparatus for detecting a displacement of a cantilever in an atomic force microscope (AFM) or the like.
2. Description of the Related Art
A cantilever displacement detection system of an atomic force microscope employs, for example, an interference method or an optical lever method. According to the interference method, a laser beam is radiated on a mirror provided on that surface of a free end portion of a cantilever, which is opposite to the surface thereof provided with a probe, and a variation in intensity of reflected beam due to a displacement of the cantilever is detected by an interferometer. On the other hand, according to the optical lever method, a variation in incidence angle of a laser beam incident on a mirror, which results from a displacement of a cantilever, is detected as an increased displacement of a beam incident on the surface of a beam receiver on the basis of the principle of optical lever.
FIG. 4 shows a conventional cantilever displacement detection apparatus employing the optical lever method. A cantilever 112 supported on a box-shaped frame 110 has a probe 114 at its free end portion. A mirror 116 is provided on that surface of the cantilever which is opposite to the surface thereof provided with the probe 114. A sample 118 is mounted on a sample table 120, such that the sample 118 faces the probe 114 at a short distance. The sample 118 is moved in an XY-direction, i.e. a direction parallel to the surface of the sample 118. The frame 110 contains a laser diode 122 for emitting a laser beam to the mirror 116, and a two-part light receiver 124 for receiving the beam reflected by the mirror 116. The light receiver 124 has two light receiving regions 124a and 124b, and the receiver 124 is adjusted such that the center of the laser beam reflected by the 116 is located at a boundary between the two light-receiving regions 124a and 124b when the cantilever 112 is in its normal measurement position (a horizontal position in FIG. 4). The light receiving regions 124a and 124b output voltage signals corresponding to the intensity of received light. By finding a difference between the two voltage signals, an inclination of the mirror 116, i.e. a displacement of the cantilever 112, can be measured.
When a sample of a living body is observed, the same observation can be performed with a sample region including the cantilever filled with a liquid, as indicated by broken line B in FIG. 4.
The detection sensitivity S of the optical lever method is expressed by EQU S=D/.DELTA.=2L/1
where D is the displacement of the beam on the light receiving surface of the two-part light receiver 124, l is the length of the cantilever 112 (normally 100 to 200 .mu.m),
L is the light path length of the reflected beam (the distance between the mirror 116 and the two-part light receiver 124), and
.DELTA. is the displacement of the probe 114
When L=100 mm and l=200 .mu.m, EQU S=200/(200.times.10.sup.-3)=10.sup.3
As has been described above, according to the optical ever method, the displacement detection system with simple structure and high sensitivity can be obtained.
A scanning tunneling microscope (STM) or an atomic force microscope (AFM) has a resolution of 10 nm to 0.1 nm which is much higher than a sub-micron resolution of an optical microscope. By the STM or AFM, a real image of a DNA in a cell structure or a molecular/atomic arrangement can be obtained.
In the early stage of development of STMs and AFMs, a crystal having an orderly atomic arrangement was employed as a sample. Any point of such a sample could be observed by simply approaching the probe to that point. However, a sample such as a cluster having an upper pattern on an orderly atomic arrangement, DNA, or a cell membrane of a living body, cannot be observed by such a method. In many cases, however, the presence of such a sample can be detected by an optical microscope although the structure thereof cannot be resolved. Thus, in order to generally locate the measurement area for the STM or AFM, it is very useful to combine an optical microscope with the STM or AFM.
A technique for combining only a probe with an
optical path in an STM was already disclosed in PCT International Publication No. W089/01603 (U.S. patent application Ser. No. 07/460,076 filed Feb. 5, 1990). A similar technique for an AFM was disclosed in Published Unexamined Japanese Patent Application H2-281103. According to this technique, a displacement of a cantilever is detected by a critical angle focus-state detection method combined integrally with an optical system. However, the sensitivity in this critical angle method is inferior to that in the optical lever method.
Accordingly, it is desirable to apply the optical lever method to the cantilever displacement detection system of the AFM. In the optical lever method, it is desirable, as shown in FIG. 4, that the laser diode and two-part light receiver be situated above the cantilever so that the incidence angle of a laser beam may not increase. If the incidence angle of the laser beam incident on the cantilever increases, the detection sensitivity would deteriorate and the detection error would increase. However, when the cantilever displacement detection optical system based on the optical lever method is applied to the AFM having an observation optical system, it is difficult to obtain a high-quality optical path because of the presence of an objective lens or the like of the observation optical system, that is, it is difficult to arrange the laser diode and two part light receiver so as not to increase the incidence angle of the laser beam.